Some of the dart2js output isn't minified because it's emitted as text strings. We're currently working on fixing that. You should save very little from minifying that.
As you may know, in JavaScript semi colons are somewhat optional, and are sometimes inserted when there is a new line. Dart2js takes advantage of this feature and emits new lines instead of semicolons. This is roughly equivalent, but is much easier to debug.
If you find that there is a tool that does a better job than dart2js itself, please report a bug at
http://dartbug.com/new.
Cheers,
Peter